The interaction of Pneumocystis carinii with rodent cells and opsonins will be investigated. Pneumocystis will be obtained by bronchial lavage of steroid treated Sprague-Dawley rats. Antipneumocystis immune serum will be produced in rabbits using lyophilized Pneumocystis. The interaction of Pneumocystis and alveolar macrophages, peritoneal macrophages, and opsonins will be investigated by phase contrast microscopy in cell chambers and fixed preparations. Methods to modify the interaction of Pneumocystis and macrophages will be studied. Differences in the ability of alveolar macrophages and peritoneal macrophages to control Pneumocystis infection will be studied.